Stroke is a leading cause of chronic disability in the United States, with nearly 610,000 new cases annually. In chronic stroke patients, slow, exhausting walking is a primary deleterious outcome that limits activities of daily living. Furthermore, impaired walking can lead to secondary impairments such as muscle weakness and poor balance, both risk factors for falls. Therefore, interventions designed to improve walking function post-stroke could have significant positive impact on the quality of life of millions. In healthy walking, the ankle extensor muscles play a major role in key walking sub-tasks including (1) propulsion (2) body-weight support and (3) swing initiation. Chronic stroke patients typically have problems with each of these sub-tasks, likely due to weak and uncoordinated ankle mechanical function in their paretic limb. Impaired unilateral ankle function results in highly asymmetric gait that limits top walking speed and elevates metabolic energy expenditure. The primary objective of the proposed research is to use a wearable robotic device to assist the paretic limb in patients with post-stroke hemiplegia and improve both gait symmetry and economy. We will recruit 20 patients with chronic stroke and construct lightweight robotic ankle exoskeletons for their paretic limb. Then we will test whether robotic assistance focused on ankle joint extension can improve paretic limb function - driving it towards that of size/age paired healthy controls. We hypothesize that targeted, unilateral ankle joint mechanical assistance will restore gait symmetry across multiple scales of musculoskeletal organization - from limbs to lower-limb joints- and reduce the metabolic cost of walking in people post-stroke. If robotic ankle exoskeletons can restore normal walking mechanics and reduce metabolic cost, then people with severe impairments post-stroke could benefit from a portable version as a permanent aid during community ambulation.